A diesel particulate filter (hereinafter referred to as “DPF”), for example, is known as a filter for collecting particulate matter (hereinafter referred to as “PM”) in an exhaust gas emitted from a diesel engine.
Also, a selective catalytic reduction (hereinafter referred to as “SCR”) catalyst is known as a nitrogen compound (hereinafter referred to as “NOx”) catalyst for purifying NOx in an exhaust gas. The SCR catalyst selectively reduces and purifies NOx in the exhaust gas by using ammonia (NH3) produced from a urea solution (urea water) upon hydrolysis with heat of the exhaust gas. In the SCR catalyst, in particular when the exhaust gas temperature is low, NOx (nitrogen monoxide or NO, and nitrogen dioxide or NO2) in the exhaust gas reacts with ammonia (NH3) according to the following chemical equation and is purified.2NH3+NO+NO2→2N2+3H2O  Equation (1)
In order to improve the purification rate of NOx in the SCR catalyst, it is preferred that the ratio of NO to NO2 be kept at 1 to 1.